FM headers for message routing

ISC message routing information is provided within SNA-defined function management (FM) headers.

On input, IMS automatically processes the information in these FM headers and then removes them from the input message. IMS then passes the message to the appropriate input edit process. On output, IMS automatically builds the necessary routing FM headers based upon information provided by IMS system definition, session bind parameters, the input FM header, or MFS.

For this explanation of distributed transaction processing, the portion of the ATTACH and SCHEDULER FM headers of interest are the routing parameters that they carry. Four fields are used to route messages to appropriate control blocks. These routing fields, present in both headers, are:

DPN (Destination Process Name): The DPN parameter names an input process to be invoked synchronously to the session. Within IMS, the processes that can be invoked are basic edit, ISC edit, or MFS. For MFS, the input DPN value is the MFS message input descriptor (MID) name. For other subsystems, the named process can be the message destination (such as a transaction code).
PRN (Primary Resource Name): The PRN parameter names a resource to be associated with the attached process. This parameter occurs on a reply returned as the result of processing a message in a remote subsystem.
Within IMS, the input PRN can name either an LTERM message queue for terminal output (IMS message switch) or an input transaction message queue. When the PRN is not available on input, IMS defaults to the normal method of using the message destination contained in the first data field (or optionally inserted by MFS formatting) or a preset transaction code established by an IMS operator command.
RDPN (Return Destination Process Name): The RDPN parameter defines a suggested return destination process name to be associated with an output message. It should be returned as the DPN on resulting message replies to facilitate the processing of replies returned to the source subsystem. The input RDPN is discarded when replies or message switch output is sent on another session. The RDPN is sent by IMS only for MFS-formatted output, wherein the RDPN is the optional chained MID name specified on the MFS message output descriptor (MOD).
RPRN (Return Primary Resource Name): The RPRN parameter defines a suggested return primary resource name to be associated with an output message. It should be returned on resulting message replies to the subsystem that was the source of the message to facilitate return reply processing within that subsystem. The input RPRN is discarded when replies or message switch output is sent on another session. The RPRN can optionally be set by use of MFS formatted output if specified on the MFS MOD.